The present invention relates primarily to the art of inductive heating and, more particularly, to an apparatus for inductively heating and then quench hardening and tempering a circumferential surface portion of a workpiece.
The invention is particularly applicable to inductively heating and then quench hardening a propeller shaft, and it will be described with particular reference thereto; however, it should be appreciated that the invention has much broader applications and may be used for inductively heating and quench hardening various other workpieces.
In the production of steel propeller shafts such as are used in motor vehicles to transmit power from the transmission output shaft to the differential, it is customary to harden the cylindrical surface portions of the shaft to impart toughness and uniform surface characteristics thereto. One of the most commonly employed methods for hardening these shaft surface portions is by inductively heating them to a temperature (generally around 1750.degree. F.) above the critical range of the particular steel of which the shaft is made, while the shaft is rotated about its central axis, and then rapidly cooling the heated surface portion of the rotating shaft by directing a stream or jet of coolant or quench fluid, such as a water-based quenching oil, from an overhead quench head downwardly onto the top of the heated surface portion of the rotating shaft. This quenching, however, of the heated surface portion of the shaft sets up hardening strains therein and causes surface brittleness which is apt to cause cracking. To relieve these hardening strains and reduce the surface brittleness, the quench hardened surface portion of the rotating shaft is then inductively reheated promptly to a somewhat lower temperature, e.g., from 800.degree. to 1000.degree. F., to temper or "draw back" the hardness of the quench hardened surface portion and increase its toughness.
With the prior quenching arrangements as described above having its quench head located directly above the heated surface portion of the propeller shaft, the drippage of residual quench fluid from the overlying quench head or nozzle end, which normally occurs following the shut-off of the supply of quench fluid to the quench head to terminate the quenching operation, would drop onto and contact the quench-hardened shaft surface portion during the subsequent reheating and tempering thereof. This caused the formation of so-called hard spots or incompletely tempered surface areas in the final quench hardened and tempered circumferential surface portions of the shaft. The resulting treated surface portions of the shaft thus possessed undesirable non-uniform surface characteristics.